The present invention relates generally to Position Sensitive Photo-Multiplier Tubes (PS-PMTs), and more specifically to a compact, modular base for a PS-PMT.
A Position Sensitive Photo-Multiplier Tube (PS-PMT) is a photosensitive device that converts light photons into an electrical current. The main components of a PS-PMT are an input window, a photocathode, focusing electrodes, dynodes and at least one anode (output). The photocathode is used for converting incoming light (photons) into electrons. These photoelectrons, which are a product of photoelectric effect, are directed by the potential of focusing electrodes towards dynodes. The dynodes are used to multiply the electrons by the process of secondary electron emission. Electron gains of 103 to 108 are common and depend on the number of dynodes and inter-dynode potentials. Dynodes are made of or covered with a layer of secondary emissive material. The condition of the dynode surfaces are responsible for PS-PMT stable gain performance. All known dynode emissive materials are sensitive to electron stress. The most sensitive dynodes are those that are at the end of the stages of dynodes, where the quantity of secondary electrons emitted is the largest. Understandably, for long-term, stable operation of a PS-PMT, a low anode current is preferable.
The voltages that create the electrostatic fields between the photocathode, the focusing electrodes and the dynodes are delivered from a single high-voltage stable power supply and a voltage divider. The divider is a common part of a PS-PMT base. The design of the divider circuit is crucial to getting the best performance from the PS-PMT. There are many versions of PS-PMT high voltage dividers optimized or designed for some particular application. Most of them are concentrated on specific parameters that are critical for a given application, such as maximum gain, dynamic range, low noise, or linearity. Series-regulator type high voltage power supplies optimized for photomultiplier tubes are well known in the art and have gained a good reputation. Other components found in or required by scintillation cameras, PS-PS-PMTs, are described in xe2x80x9cPhotomultiplier Tube, Principle to Applicationxe2x80x9d by Hamamatsu Photonics K. K., March 1994, which is incorporated herein by reference.
The output of a photomultiplier tube is a current (charge), while the external signal processing circuits are usually designed to handle a voltage signal. Therefore, the current output must be converted into a voltage signal by a current to voltage converter. Further, the current that is output from a PS-PMT anode is very small, especially in low light level detection, low gain PS-PMT""s, and photon counting applications. An operational amplifier can be used to both convert the anode output current to a voltage and accurately amplify the resulting voltage. Typically this operational amplifier is powered by a source that is separate from the high voltage power source for the dynode stages of the PS-PMT. This is done to insure the stability of the power supply to the dynodes.
Many PS-PMTs have multiple anodes that are usually arranged in X and Y arrays to provide accurate imaging capabilities. The analog signal outputs from the anodes can be processed individually or combined in a variety of ways and the results analyzed using appropriate data acquisition systems under computer control. The processed data can then be displayed on a video monitor for further study of the subject being imaged. The need for improved image resolution and/or larger imaging area of PS-PMTs requires increasing the number of anode electrodes, which is limited by technology developments in the area of fabrication of photomultiplier devices. Alternatively, the imaging area can be increased by just-a-posing individual PS-PMTs in the form of arrays and matrices. However, this method has many draw backs including the possibility of overlapping or creating gaps in the imaging area. Therefore, improving image resolution and/or providing a larger imaging area of PS-PMTs requires increasing the number of anode electrodes.
Individual anode electrodes are normally connected to sensitive signal amplifiers with appropriate specifications for signal bandwidth, noise and gain. Because of electrical performance considerations, such analog instrumentation is usually placed as close as physically permissible to the anode electrodes. However, as the number of electrodes increase or the size of the PS-PMT decreases, the instrumentation required by each individual electrode becomes prohibitive due to physical and/or cost constraints.
In order to overcome some of the physical and cost limitations, caused by the instrumentation electronics associated with each anode electrode, a resistive divider readout technique can be employed. Because the anode electrodes in a PS-PMT are functionally identical to a current source, anode electrodes for the same imaging coordinate can be interconnected in a chain by means of resistors. The last anode electrode on each end of the resistor chains is then connected to a load resistor, and the signal developed across this load can be amplified as required. By characterizing the analog signals from each end of the chain, it is possible to determine the position of occurrence of photon events along the interconnected chain of anode electrodes. Thus, the number of analog signal channels with resistive divider readout is independent of the total number of anode electrodes and is reduced to two channels per coordinate; two X outputs and two Y outputs.
Applications employing the use of multiple PS-PMTs to cover larger imaging areas, than a single PS-PMT, are usually complex, costly and specific to the requirements imposed by the implementation. Such applications have been described in Koji Inoue, et al., xe2x80x9cNuclear Instruments and Methodsxe2x80x9d, A 423 (1999) pp. 364-368. If still larger imaging areas are required, the number of electronic channels is correspondingly increased or a new and specific implementation is needed.
The present concept is to provide an array of PS-PMTs that is simple, modular and non-specific. This is achieved by providing a user-configurable electronics base that connects to a single PS-PMT and contains circuitry for high voltage biasing, dynode signal extraction and amplification for fast trigger qualification, resistor chains for each of the X and Y coordinates, signal amplifiers and configuration jumpers. This user-configurable base can be used by itself for imaging a subject, it can be connected to one other user-configurable base to double the imaging area, or it can be connected to multiple user-configurable bases to form a matrix of PS-PMTs.
A modular base for a position sensitive photo-multiplier tube that can be used by itself and can be coupled to one or more like modular bases to create a matrix of bases. The present base provides multiple options to a user in regards to the number of bases to be used in either the X or the Y coordinates within the matrix. Each modular base comprises: resistor chains that connect the anodes in each X and Y coordinate; circuitry for high voltage biasing that provides power to the dynode stages of the PS-PMT; and, circuitry for dynode signal extraction and amplification that provides fast trigger qualification. Each base also includes two signal amplifiers for each X and Y coordinate output, for a total of four amplifiers. Both signal amplifiers for each coordinate are only used when the base is not attached to other bases. Configuration jumpers provided in each base are used to connect X and Y outputs to other modular bases.
The position sensitive photo-multiplier tube (PS-PMT) includes multiple X coordinate anode outputs, multiple Y coordinate anode outputs, and each set of multiple outputs are coupled together by the resistor chains.
It is an object of the present invention to provide a compact modular PS-PMT base that can be used alone or in combination with an unlimited number of similar bases.
It is another object of the present invention to reduce the amount of required hardware in PS-PMT arrays.
It is a further object of the present method to allow a user the freedom to specifically build an array of bases based on the required number of PS-PMT""s.